1. Field of the Invention
The invention relates to a load actuation circuit used for actuating or driving an electrical load, and more particularly to a load actuation circuit having a function of limiting the load current supplied to the electrical load within a predetermined value or a function of protecting the load actuation circuit against excessive load current.
2. Related Art
Laid-open Japanese Patent Application No. 5-327442 discloses a conventional load actuation circuit that has a function of limiting the load current when the load current becomes an excessive value.
According to the load actuation circuit disclosed in this prior art, an output MOS transistor is provided to supply load current to an electrical load. The drain and gate terminals of this output MOS transistor are connected respectively to the drain and gate terminals of a current-detecting MOS transistor. A current-controlling NPN transistor is connected to the source of the current-detecting transistor. When the load current flowing across the output MOS transistor becomes an excessive value, the gate voltage applied to both of the output MOS transistor and the current-detecting MOS transistor is suppressed by the control of this NPN transistor. Thus, the current flowing across the output MOS transistor is limited within a predetermined value.
However, according to the above-described conventional arrangement, only one NPN transistor is used to perform a current-limit operation to suppress the excessive load current. Accordingly, there is a problem that the temperature characteristics of the NPN transistor may deteriorate the accuracy in the current-limit operation.
Provision of the NPN transistor causes a difference between the gate-source voltage of the output MOS transistor and the gate-source voltage of the current-detecting MOS transistor. Such a difference usually differentiates the operating point of the output MOS from the operating point of the current-detecting MOS transistor. This possibly results in an unstable current-detecting operation.